1. Field of the Invention
The invention relates to a hydraulic drive for displacing an actuator between two predetermined end positions, comprising a piston unit which can be pressurized in a cylinder unit in opposite directions by way of hydraulic springs and a control device for alternating pressurization in opposite directions of the piston unit.
2. Description of the Prior Art
In order to enable the use of the energy applied via a hydraulic drive for opening a valve of an internal combustion engine also for closing the valve it is known (EP 1 215 369 A2) to use the compressibility of the hydraulic medium for forming two hydraulic springs for a pressurization in opposite directions of a piston, so that the energy applied via a hydraulic spring onto the piston is stored in the other hydraulic spring apart from frictional and leakage losses in order to be available for pressurizing the piston in opposite directions. The piston with the valve body to be driven thus forms a free oscillator in combination with the two hydraulic springs, which oscillator is held back or released in the two reversing positions by a control device. For this purpose, a control valve is used in a pressurization line joining the hydraulic spring with a pressure accumulator. The control valve is closed in the two end positions of the piston in order to move the piston, during the opening, via the respectively tensioned hydraulic spring under a tension of the other hydraulic spring to the opposite end position where the control valve is closed again for holding back the piston. Although actuators can be displaced with a comparatively low energy input between two end positions at high speed with the help of this known hydraulic drive, since it is merely necessary to compensate frictional and leakage losses, the movement of the actuator also depends on the switching speed of the control valve. Moreover, a separate control intervention is necessary for holding back the piston in the two reversing positions.
The invention is thus based on the object of improving a hydraulic drive of the kind mentioned above with simple constructional means in such a way that a movement of the actuator can be ensured which is independent of the actuating speed of the control device, and that for holding back the piston unit in the reversing positions no separate control intervention is required.